We are attempting to understand the principle of chromosome organization by studying the organization of nucleotide sequences in chromosomal DNA, in particular, that of Drosophila. We are continuing these studies in three ways: 1) by isolating folded rings from chromosomal DNA fragments; 2) by isolating palindromic DNA, and those sequences flanking the palindromes; 3) by fractionating restriction endonuclease segments with respect to length and composition. By appropriate combinations of these experiments, we intend to isolate and purify discrete sequences from chromosomal DNA. In this way we hope to study the organization of nucleotide sequences corresponding to single eukaryotic genes.